1. Field of the Invention
The present invention relates to an improvement of a swirl flow type jet mill in which solids are milled by using the energy of compressed air. More particularly, the invention relates to a micromilling device in which energy consumption during milling is reduced and the milled particle size distribution is narrowed.
2. Discussion of the Related Art
In a conventional swirl flow type jet mill which possesses a swirl milling chamber (which will be referred to below simply as a "jet mill"), compressed air is injected into the milling chamber from milling nozzles. The energy of the high-speed air stream causes collisions between solid particles, thereby milling a solid. The swirl flow produced by the high-speed air stream brings about centrifugal classification of the particles, resulting in particles with the required particle diameter.
Advantages of a jet mill include the fact that it is possible to mill solids that are sensitive to heat since injection of compressed air lowers the temperature due to an adiabatic expansion effect. Further, jet mills are suited to perform micromilling, since the milling is effected mainly through surface milling by collisions between particles.
On the other hand, a jet mill has several drawbacks. Since a large amount of compressed air is used, it is necessary to have a large compressor. Thereby the amount of energy consumed in milling by a jet mill is a very large quantity, some 2-5 times the amount consumed in a mechanical mill. Moreover, since collisions between particles are the main feature of the jet mill, ultrafine powder is liable to be produced, while other particles which have taken part in few collisions are discharged directly as coarse powder. This results in a broad milled particle size distribution.
An attempt to get a narrow milled particle size distribution has been proposed. This attempt involves the combined use of a classifier with a jet mill and the adoption of a closed-circuit milling system in which a material is milled in a jet mill to particles whose diameter is larger than the required diameter. These are classified by the coarse powder classifier, and coarse powder is returned to the mill again to produce the required particle diameter. This system makes it possible to mill material with a considerable narrow particle size distribution. However, this system is unable to provide any improvement with respect to the energy consumed in milling.
To reduce the energy consumption, the mill disclosed in Japanese Laid-open Utility Model Application Nos. 51-1000374, 51-1000375 and 56-64754 and Japanese Laid-open Patent Application No. 58-143853 are proposed. These mills employ systems in which there is a single milling nozzle for a single impact plate. These systems provide some improvement with respect to the energy consumed by milling. However, there is the drawback that if these mills are used alone the milled particle size distribution is greater than it is with a jet mill. These systems require a combination with a classifier. These systems have an added drawback that as single nozzles are employed, it is not possible to make the equipment large. Employing a single nozzle in large equipment would result in a drop in efficiency. The mill disclosed in Japanese Laid-open Patent Application No. 57-84756 has communication pipes, and so the structure becomes complex and the equipment is unpractical if there is a large number of milling nozzles.